1. Field of the Invention
The present invention relates to a three-way electromagnetic valve which is suitable for controlling a high pressure fluid and, for example, suitable for use in a fuel injection system of a diesel engine.
2. Description of the Related Art
Heretofore, a diesel engine employs an injector which injects a high pressure fuel supplied from, for example, a common rail to a cylinder through a nozzle opened and closed by a nozzle needle, and there has been known a three-way electromagnetic valve which is mounted in such an injector to control the seat and lift of the nozzle needle by changing over a back pressure acting on the nozzle needle between high and low levels.
An example of this kind of three-way electromagnetic valve is disclosed in, for example, Japanese Patent Unexamined Publication No. 2-253072, in which valve provides with a sliding portion shown in FIG. 7.
A sliding portion 410 of the conventional three-way electromagnetic valve will be described in a schematic structure thereof with reference to FIG. 7.
A valve body 412 is a cylindrical body provided with an outer valve sliding hole 414 formed by boring in the central portion thereof. The valve body 412 is further provided therein with an inlet passage 416 through which a high pressure fluid is introduced into the sliding portion 410 of the three-way electromagnetic valve, a supply port 418 opened in the wall surface of the outer valve sliding hole 414 so as to be communicated with the inlet passage 416, a groove 420 annularly formed by boring in the wall surface of the outer valve sliding hole 414 so as to be communicated with the supply port 418, a discharge port 422 formed by boring so as to be communicated with the lower end portion of the outer valve sliding hole 414, a discharge passage 424 formed in communication with the discharge port 422, a funnel-shaped valve seat 426 formed in communication with the lower end portion of the outer valve sliding hole 414 so as to be substantially coaxial with the outer valve sliding hole 414, and a control port 428 communicated with the outer valve sliding hole 414 through the valve seat 426.
into the outer valve sliding hole 414 is slidably inserted a cylindrical outer valve 434 which has a poppet portion 430 formed at the lower end portion thereof and a flange portion 432 formed at the upper end portion thereof. The outer valve 434 can be moved relatively in the axial direction thereof so that the poppet portion 430 can be brought into contact with and moved apart from the valve seat 426 of the valve body 412, thereby making it possible to stop and provide communication between the discharge port 422 and the control port 428 in response to the contact and departure of the poppet portion 430 with and from the valve seat 426.
The outer valve 434 is provided therein with an inner valve sliding hole 436 formed by boring so as to be opened at the flange portion 432, an inner chamber 438 formed in the central portion thereof so as to be continuously connected with the inner valve sliding hole 436, a funnel-shaped inner seat 440 which is formed substantially coaxially with the inner valve sliding hole 436 so as to be continuously connected with the inner cheer 438, a passage 442 communicated with the inner chamber 438 through the inner seat 440, and a plurality of through holes 444 which are formed by boring substantially at equal angular intervals so as to extend in the radial direction of the inner sliding hole 436 for serving to enable the inner chamber 438 and the groove 420 of the outer valve sliding hole 414 to communicate with each other. Incidentally, an internal path 445 is formed by following from the inner chamber 438 to the through holes 444.
An inner valve 446 of a stepped cylindrical shape in which the diameter of the lower end portion thereof is small and the diameter of the central portion thereof is large is slidably inserted in the inner valve sliding hole 436. The central portion of the inner valve 446 serves to form a sliding shaft portion 448 and the lower end portion thereof serves to form a poppet portion 450. By changing the relative position of the inner valve 446 and the outer valve 434 in the axial direction thereof, the poppet portion 450 can be brought into contact with and moved apart from the inner seat 440, thereby making it possible to stop and provide communication between the inner chamber 438 of the outer valve 434 and the passage 442 in response to the contact and departure of the poppet portion 450 with and from the inner seat 440.
With such construction of the sliding portion 410, when the outer valve 434 is seated on the valve seat 426 to make the inner valve move apart from the inner seat 440, the high pressure fuel supplied through the inlet passage 416 connected to, for example, a common rail can be introduced into the control port 428 via the supply port 418 and the internal path 445, while, when the outer valve 434 is moved apart from the valve seat 426 to make the inner valve 446 seat on the inner seat 440, the high pressure fuel supplied through the inlet passage 416 is made to stop flowing so that the fuel is guided from the control port 428 to the discharge port 422.
In the sliding portion 410 of such conventional three-way electromagnetic valve, the clearance between the outer valve sliding hole 414 and the outer valve 434 and the clearance between the inner valve sliding hole 436 and the inner valve 446 are set to be about 1 to 2 .mu.m in terms of diameter for the purpose of keeping the fuel leakage at a minimum to thereby prevent the loss of driving torque of, for example, the fuel pump and a reduction in the fuel injection pressure. Accordingly, the valve body 412 and the outer valve 434 as well as the outer valve 434 and the inner valve 446 are made to slide on each other precisely.
However, since the pressure of the high pressure fuel introduced into the sliding portion 410 of the three-way electromagnetic valve is as high as about 120 MPa, the valve body 412 is deformed outwardly in the radial direction in the vicinity of the groove 420 due to the effect of the high pressure fuel introduced into the groove 420. Such deformation of the valve body 412 causes the clearance between the outer valve sliding hole 414 and the outer valve 434 to be increased. Therefore, the high pressure fuel leaks out into the increased clearance and acts to pressurize the whole of the wall surface of the outer valve sliding hole 414, with a result that the valve body 412 is deformed outwardly. FIG. 8 shows an example of the measured values of the extent of deformation of the valve body 412. In FIG. 8, the extent of deformation of the valve body 412 is 1.97.times.10.sup.-3 mm in the vicinity of the groove 420 and 2.80.times.10.sup.-4 mm in the vicinity of the discharge port 422. Accordingly, as shown by the broken line, the valve body 412 is deformed such that the extent of deformation thereof is large in the vicinity of the groove 420 but small at the upper and lower end portions of the outer valve sliding hole 414.
Accordingly, part of the high pressure fuel flowing through the supply port 418 into the three-way electromagnetic valve 400 passes through the clearance enlarged by the above deformation of the valve body 412 and leaks out through the groove 420 toward the upper and lower end portions of the outer valve sliding hole 414. In this case, there is a possibility that a foreign matter contained in the high pressure fuel, such as fine chip, cannot pass through the portion with small clearance at the upper and lower end portions of the outer valve sliding hole 414 to thereby be filled with therein, though it can pass through the portion with large clearance in the vicinity of the groove 420, so that it accumulates in the clearance. Such accumulation of the foreign matter gives rise to a problem of bad sliding, sticking and the like of the outer valve.